Transformers are an important component used in radio frequency (RF) circuitry. They can be used in filter circuits, in impedance matching circuits, and in transforming balanced to unbalanced (balun) circuits. Lower RF applications (low hundreds of megahertz (MHZ) traditionally use windings on a ferrite core, with the square of the ratio of primary to secondary windings (Np/Ns)2 representing an impedance ratio (Zp/Zs). The power is transferred through the ferrite core. Higher RF applications (high hundreds of MHz to low gigahertz (GHz) often use transmission line transformers. The transmission lines may be implemented as coaxial cables or twisted enameled wires. In this case, the power is transferred through the dielectric medium of the transmission line. The characteristic impedance of the transmission line is critical in obtaining optimum performance of the transformer. In order to extend the lower frequency operation of transmission line transformers, the transmission line or lines may be wound on a ferrite core.
Further still, in order to achieve certain impedance transformations using transmission line transformers, the output of one or more transmission lines may be connected to the input of one or more transmission lines. Such a case is shown in FIG. 1, which shows the connections necessary to achieve a 9:1 impedance transformation. The transmission line transformer of FIG. 1 is made up of a first transmission line T1′ and a second transmission line T2′. The first transmission line T1′ is a coaxial cable having a first center conductor L1′ with a first input terminal IN1′ at one end and a first output terminal OUT1′ at another end. A first outer conductor L2′ provides electrical shielding for the first center conductor L1′. The second transmission line T2′ is another coaxial cable having a second center conductor L3′ with a second input terminal IN2′ at one end and a second output terminal OUT2′ at another end. A second outer conductor L4′ provides electrical shielding for the second center conductor L3′.
A first electrical interconnector E1′ couples a first point P1′ located on the first outer conductor L2′ proximal to the first input terminal IN1′ to a second point P2′ located on the second outer conductor L4′ proximal to the second output terminal OUT2′. A second electrical interconnector E2′ couples a third point P3′ located on the second outer conductor L4′ proximal to the second input terminal IN2′ to a fourth point P4′ located on the first outer conductor L2′ proximal to the first output terminal OUT1′. A third electrical interconnector E3′ couples the second point P2′ to a fifth point P5′ that is electrically common with the first output terminal OUT1′. A fourth electrical interconnector E4′ couples the fourth point P4′ to a sixth point P6′ that is electrically common with the second output terminal OUT2′. In this particular configuration, an input load impedance RL is transformed to an output load impedance of RL/9, which is a 9:1 impedance transformation ratio. Other electrical interconnections provide different impedance ratios, such as 2.25:1, 4:1, and 6.25:1.
As a frequency of operation increases, it is important to make the connections shown in FIG. 1 as short as possible. Otherwise, parasitic properties of these connections will degrade the performance of the transformer. For this reason, the first transmission line T1′ and the second transmission line T2′ are often bent into a U-shape. Alternatively, if a first enamel-coated wire transmission line (not shown) and a second enamel-coated wire transmission line are twisted and wound on a ferrite core, the resulting windings are configured such that input and output connections of the first enamel-coated wire transmission line and the second enamel-coated wire transmission line are proximal to each other.
Transmission line transformers implemented using coaxial cable or twisted wire are relatively large compared with the physical size of typical handset power amplifiers and many other high-volume consumer electronics components. In addition, such implementations, especially if the implementation uses a ferrite core, are relatively costly. Since the ferrite core is used only to extend the low-frequency operation, and since the transmission line transformer transfers power through the dielectric medium, an implementation of such a transformer without a ferrite core is reasonable for handset power amplifier applications where a very broad bandwidth (e.g., a multi-octave bandwidth) is not required. However, as alluded to above, the physical size of coaxial cable is not suitable for integration into RF circuitry of wireless handsets such as smartphones. On the other hand, RF circuitry using printed circuit technology typically implements transmission lines as microstrip lines that are planar and take up a relatively small amount of space.
A schematic of a microstrip line MS1 is shown in FIG. 2. A number of papers in literature describe implementations of transmission line transformers using microstrip lines. As such, microstrip lines are usable to construct compact impedance transformers. However, traditional compact impedance transformers constructed using microstrip lines typically have a relatively difficult electrical connection methodology in comparison to typical transmission line transformers such as the one depicted in FIG. 1. A difficulty in using microstrip lines lies primarily in the fact that a return current path is a ground plane GP under the microstrip that is common to all transmission lines.
Referring back to FIG. 1, the input return current point of one transmission line cannot be distinctly connected to the output return current point of another transmission line because these connection points are already part of the ground plane. This difficulty applies particularly to transformers that require connections between the input and output terminals of the transformer. What is needed is a compact impedance transformer that does not require a common ground plane for output return current points.